Police say an adult was also taken to hospital - alongside eight children - following a suspected poisoning incident at a summer camp in Leicestershire on Sunday.
The update comes as officers continue to question a 76-year-old man, arrested on Monday evening at a nearby pub on suspicion of administering poison or a noxious thing with intent to injure, aggrieve or annoy.
In the latest clarification of what happened at the camp in the village of Stathern, Leicestershire Police said on Friday that the adult was taken to hospital as a precaution and discharged.
Leicestershire Police originally said the incident happened on Monday, before amending it to Sunday, and it is still unclear whether officers only responded on Monday, and whether that is why the force has referred itself to the Independent Office for Police Conduct (IOPC).
The East Midlands Ambulance Service said on Thursday it had taken nine patients to hospital after getting a call at 4pm on Monday about a medical emergency at an address in Plungar, Nottingham.
Ten crewed ambulances, along with paramedics and responders from the air ambulance were sent to the incident, it said.
All eight children taken to hospital as a precaution have since been discharged, and their parents or guardians have been contacted.
Detective Inspector Neil Holden, the senior investigating officer, said: "We understand the concern this incident will have caused to parents, guardians and the surrounding community.
He added: "This is a complex and sensitive investigation and we will continue to provide updates to both parents and guardians and the public as and when we can."
The IOPC said: "Our assessment team has examined all available evidence and concluded the matter should be independently investigated by the IOPC.
"The investigation will look at whether there were any breaches of professional behaviour - namely a failure to carry out duties and responsibilities - that resulted in a delay in Leicestershire Police's response to what was later declared a critical incident."
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